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Possibly there are a few more words about slaves because the relationships within family are already settled matters in the Old Testament. There are no instructions in the Old Testament that I have noticed about how slaves should behave with respect to their masters. There is another dimension to this though that makes the slave/master issue important.

If you take the 'fullness in Christ' idea along with the 'useless human rules' idea, you create a potential temptation for the slave.
1)Since "in Christ you have been brought to fullness" (Colossians 2:10) and
2)"Since you died with Christ to the elemental spiritual forces of this world, why, as though you still belonged to the world, do you submit to its rules" (Colossians 2:20)

The slave may think like this. 'Well since in Christ I am a king and since my master is a slave to Satan and trying to force me to live a useless life under human laws, why should I obey my huge inferior? I could run away and preach the gospel to sinners afar?' For the sake of love, Paul wants to teach slaves that they are free and only serve God, but to love their masters with respect. In this way the slave is free from horrible feelings of inferiority and injustice, but in his freedom he can serve God by respecting his inferior. In this way also, evangelism starts by letting our beattitudes shine in the place God has put us.

How simple and glorious are the ethics of grace. Warms the heart, eh? If I was a slave to a very harsh master, I would have been brought to tears hearing this gospel.